Adeno-associated virus (AAV) vectors have been explored extensively in numerous pre-clinical studies with varying degrees of success. A numberof these pre-clinical studies are now translating into encouraging results in phase l/ll clinical trials. In fact most recently, self-complementary (sc)AAV vectors have demonstrated sustained and potentially therapeutic levels of Factor IX in Phase I study of hemophilic B patients negative for pre-existing /\AV Ab (ASH Nov 2010 &ASCGT 2011). With 90% ofthe human population naturally infected by AAV2, and 30-50% expressing NAb, contiuned success will require resolution of this problem. For these reasons Project 1 has focused on role of N/\b in AAVFIX gene transfer and describes a proposal to better understand the biology of Ab response to AAV capsids and to engineer novel molecular approaches to overcome these limitations. HLA class II phenotypes play a major role in antigen presentation to induce antibody production. Although almost every individual produces AAV specific immunoglobulin, NAb is only detectable in half of the population, indicating that not all /\AV-specific Abs produced possess neutralizing activity. To determine which HLA class II phenotypes are associated with AAV NAb production, we will screen the HLA-class II phenotypes, AAV specific immunoglobulins and AAV NAb profile in large number of human subjects (Aim 1). HLA-class II molecules bind to peptides of 12 to 18 aa and present the complex on antigen presenting cells to induce a humoral immune response. To further elucidate w/hich epitopes from AAV capsid are bound by NAb, we will use sera from St. Jude FIX trial (see let of support, Nathawni), AAV immunized HLA-class II transgenic mice and humans with NAb positive serum for this study (Aim 2). There are several approaches being considered to evade /\AV NAb including chemical and genetic modification of AAV virion in the presence of neutralizing antibodies or randomly mutating AAV capsid. Based on the information from eiptope mapping, a novel aptamer selection against Ab will be explored (Aim 3). The broad objective of this proposal is to advance our understanding of capsid antigen presentation and examine therapies to circumvent major limitations imposed by systemic humoral immunity. RELEVANCE (See instructions): Program Project 1 objective is to better understand /V^V capsid antigen biology in novel animal models of bleeding disorders and explore molecular approaches in hope of extending the clinical success seen with scAAV FIX.